A thermostatic mixing valve blends hot and cold water to hold a set outlet temperature. Commercial buildings use one to limit scalding and to let the water heater run hot, which the CDC ties to Legionella control. The right type depends on placement: at the heater (ASSE 1017) or near the fixture (ASSE 1070).
Reason one: scald protection
ASSE International's Scald Awareness Task Group put it bluntly in its white paper: "Hot water from the tap accounts for more than 25 percent of all scald burns in children. The elderly and the physically impaired are at increased risk of scald burns also." Temperature control devices exist because of that.
Reason two: store it hot, deliver it cooler
Legionella bacteria grow in a temperature range. CDC's toolkit says to "Store hot water at temperatures above 140°F (60°C)," to keep circulating hot water above 120°F (49°C), and to keep cold water below the growth range most favorable to Legionella (77 to 113°F). That water is hot enough to scald, so it has to be brought down before it reaches people. CDC's design recommendation says: "Install thermostatic mixing valves as close as possible to fixtures." The next sentence reads: "This will prevent scalding while permitting circulating hot water temperatures above 120°F (49°C)."
The ASSE white paper makes the same point about a master valve at the heater, as a side benefit. It says the valve can "allow the water to be stored at a higher temperature, extending the amount of hot water available, and also reducing the chance of legionella (ASHRAE Guideline 12)." Our page on Legionella water management plans covers the rest of that picture.
Which ASSE standard fits where
The standard number tells you where the valve belongs. ASSE's matrix and Leonard Valve's summary agree on the basics:
- ASSE 1017, at the hot water source. ASSE says these valves are "NOT intended for point of use applications," and that used alone they "do not provide thermal shock protection or adequate scald protection." Further mixing downstream is allowed for final control.
- ASSE 1070, near or integral to the fixture. These water temperature limiting devices are for fittings on sinks, lavatories, bidets or bathtubs. ASSE says the user control allows adjustment up to a maximum of 120°F, and they provide scald protection only, not thermal shock protection.
- ASSE 1016, shower and tub/shower valves. These are the pressure balancing, thermostatic or combination valves at an individual shower. See pressure balancing versus thermostatic shower valves.
ASSE warns that installing the wrong device in the wrong location "can be the same as if no temperature control device was installed." A master valve at the heater is not a substitute for protection at the fixture; ASSE's matrix says it needs additional downstream safety devices.
What the code requires
Two sections of the 2024 International Plumbing Code (IPC) matter here.
Section 607.1. In nonresidential occupancies, hot water or tempered water shall be supplied for bathing and washing purposes. Section 607.1.2 then says tempered water shall be controlled by one of four options: an ASSE 1070 limiting device set to not greater than 110°F, a thermostatic mixing valve conforming to ASSE 1017, or a water heater conforming to ASSE 1082 or ASSE 1084.
Section 412.3. Individual shower and tub-shower valves must be balanced-pressure, thermostatic or combination valves that conform to ASSE 1016 or ASME A112.18.1, installed at the point of use, with a means to limit the maximum setting to 120°F. The section adds that "In-line thermostatic valves shall not be utilized for compliance with this section."
Phoenix's published amendments to the 2024 IPC don't list Sections 607 or 412, so the base text applies to that code. The Uniform Plumbing Code has its own shower-valve section (ASSE's matrix points to UPC 408.3), so confirm with the plan reviewer which code governs your building.
They need upkeep
A TMV is a component, not a set-and-forget fix. CDC lists thermostatic mixing valves among the water system components to clean and maintain regularly, and ASSE notes that the temperature limit stop on shower valves may need periodic adjustment. If a shower's temperature keeps changing, see why shower temperature keeps changing.
HQ handles commercial water heater and hot water plumbing, is licensed and insured, and gives free estimates with upfront pricing. If you're not sure whether your building has a master valve, point-of-use valves, or neither, that's a fair thing to have looked at. Related: what temperature a water heater should be set to.